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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1115 PM CST Monday Jan 5 2009 


Update... 
/903 PM CST Monday Jan 5 2009/ 
several recent acars soundings out of Lambert show that the 
elevated warm layer has progressed far enough northeast that some 
modifications to the ptype forecast are in order across the southern County Warning Area. 
Peak temperature on the latest sounding was near 0 degc between 
700-800 mb. Precipitation thus far across Arkansas into Tennessee has shown little 
if any northward motion...although with the short wave trough just moving out 
of The Rockies into the plains expect to see the low-middle tropopause flow 
begin backing some. This should either help with a northward shift to 
the current precipitation area or result in newer development. Indeed over 
the last hour...precipitation echoes aloft have been showing up in NE OK on 
the regional radar in NE OK. While there will be some diabatic 
cooling once the precipitation begins across southeast MO and southern Illinois...the 
strength of the warm air advection will be too great to be offset based on the 
aforementioned northward progression of the elevated warm layer. This 
suggests that the bulk of the precipitation across the Eastern Ozarks into southern 
Illinois will be freezing rain and possibly some sleet early on. I have 
modified the forecast with this in mind and the expectation of a 
light glaze of ice within the advisory area. At this time no changes to the 
northward extent of the main precipitation axis or advisory...but will have to 
monitor precipitation development during the remainder of the evening and 
overnight to see if this eventually could be warranted. 


Glass 


&& 


Discussion... 
/441 PM CST Monday Jan 5 2009/ 
still aprs that area will rcv glancing blow from precipitation associated with trough 
moving through area over the next 24 hours. Nwp output continues to indicate 
that the bulk of the precipitation will remain confined to our S...in vicinity of strong 
850 mb baroclinicity...with north fringes of precipitation shield just brushing our 
southeast counties. See no reason to disagree with this assessment since 
short range models seem to be handling current precipitation trends quite well. 


Although north fringes of precipitation shield is currently well S of our forecast area...this 
edge is expected drift north overnight as trough captures energy over SW U.S. 
And it begins to lift out. Forecast soundings certainly suggesting that that 
moisture acr much of the forecast area will be stratfied in the lowest 5-7kft. 
WRF/NAM has been hinting at this for several days over the northwest 2/3 of our 
County Warning Area...and the 12z GFS is now indicating this moisture stratification as 
well. So...with lack of ice nuclei it could be precipitation in most areas 
will be a combo of drizzle or freezing drizzle and light snow. Exception to this 
scenario should be in the far southeast counties...where deeper moisture lt tonight 
and early Tuesday morning should lead to colder cloud temperatures and btr chances for 
measurable precipitation. Although quantitative precipitation forecast will be light...temperature profile is such that 
ptype in the southeast counties could be light snow...sleet...or freezing 
rain/freezing drizzle. 


Have decided to issue advisory for the southeast counties for this light wintry 
mix for the overnight and early morning hours...06z-15z. It is also not out 
of the question that the advsy may need to be expanded northward lt tonight 
and early Tuesday...depending on if and/or how quickly and how widespread freezing 
drizzle development occurs. Evening...and especially middle shifts...will have to monitor 
short term trends to determine if a further expansion is 
necessary. 


The light precipitation should end acr the entire area Tuesday night with passage of upper air 
trough and weak cold front. 


Ovrall trends of the medium range models remain fairly similar to those 
of the last few days. However...with strong baroclinicity becoming 
established over area these subtle differences culd have a sizable 
impact...especially on temperatures. On Wednesday...models are dropping secondary shtwv 
into upper msvly which will briefly delay the southward surge of the coldest 
air...and as a result MOS maximum temperatures for Wednesday have warmed considerably 
from earlier runs. Have upped going temperatures for Wednesday acr the S and west in 
response to this...but have continued to lean toward the cold side for temperatures 
in our NE where strong cold air advection will have an impact during the afternoon. After a cold 
Wednesday night...models suggesting that warmer air will return a bit more rdply 
for Thursday and again have split the difference between going forecasts and 
guidance to lessen swings. There is gd agreement in the models that Friday 
should be a very warm day...and have pushed temperatures up to near the latest MOS 
output for this period. Cold air and below normal temperatures should return in 
the wake of Friday system. 


As far as precipitation is concerned...continued slight chance probability of precipitation in our NE areas 
Wednesday near the dynamics of the aforementioned shtwv. Also continued low 
probability of precipitation in the Friday/Friday night time frame with passage of cold front. 12z GFS 
run hinted at a clipper Wednesday night but this is not supported by 18z run 
or by European model (ecmwf)...so have held off on precipitation for that time period. 


Truett/Byrd 


&& 


Aviation... 
/1059 PM CST Monday Jan 5 2009/ 
for the 06z tafs...light east-southeasterly surface winds should continue lt tgt with 
surface ridge just east of the County Warning Area. Cirrus shield across cou...stl and sus 
should expand northward with approach of positively tilted middle-upper level 
trough currently over the plains. Patchy middle level cloudiness also 
developing over central MO as low-middle level warm air advection increases over the area. More 
significant is the low level cloudiness which has spread northeastward into 
vih...dmo and familiar around 2000-3000 feet. With southwesterly low level flow this 
will spread northeastward into cou...stl and sus shortly after 06z Tuesday and 
into uin around 12z Tuesday based on latest model relative humidity forecasts. The lowest 
cloud ceiling should occur in stl/sus from 12-18z Tuesday with ceilings 
dropping to 1000-1500 feet. The northern edge of the precipitation shield will 
also spread into the stl metropolitan area by early Tuesday morning around 12z 
Tuesday and continuing until around 17z Tuesday. This precipitation should be light in the 
form of freezing drizzle and/or light snow/flurries. The surface wind will 
continue to be relatively light but should shift to a more westerly direction 
by Tuesday afternoon or evening as a weak surface low develops in the Tennessee Valley region 
and a weak surface trough moves southeastward through the region. The low level cloud 
ceiling should scattered out from west to east Tuesday afternoon and evening...but middle level 
clouds can be expected Tuesday night with the approaching middle-upper level 
trough still just west of the County Warning Area. 


Gks 


&& 


Lsx watches/warnings/advisories... 
MO...Winter Weather Advisory until 9 am CST Tuesday for Iron-Madison- 
Reynolds-St. Francois-Ste. Genevieve. 


Illinois...Winter Weather Advisory until 9 am CST Tuesday for Clinton Illinois- 
Fayette Illinois-Marion Illinois-Randolph Illinois-Washington Illinois. 


&& 


$$ 


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